It is very difficult to chemically mill titanium and other refractory metals other than the alpha alloys of titanium, because of the high affinity of beta and alpha beta alloys for hydrogen. High hydrogen absorption results in great internal stresses which tend to crack the metal. We have discovered that the combination of phosphoric acid, nitric acid, and hydrofluoric acid results in a chemical milling composition which has significant advantages over conventional etchants and the composition of Ser. No. 200,143, particularly if combined with carbamide.
In co-pending application of John Gumbelevicius Ser. No. 200,143, filed Nov. 18, 1971, now U.S. Pat. No. 3,788,914 assigned to the assignee of this application, there is described a chemical milling composition containing nitric acid, hydrofluoric acid, and carbamide which results in a substantial lowering of the hydrogen absorption in beta and alpha beta titanium.
The composition of this invention is an improvement of the composition of Ser. No. 200,143 in that it provides considerably better tolerances in the chemically milled workpiece as well as causing even less hydrogen absorption in beta titanium and alpha beta titanium during chemical milling.
Another advantage of this invention is that it provides an even, controlled etch rate with a reduction of tapering and grooving problems usually associated with the use of conventional nitric-hydrofluoric acid mixtures. This significantly improves the operation of and tolerance control in chemical milling procedures. Another advantage of this composition is an improved solution life which results from the production of an easily removable, considerably softer, sludge-like scale in the etch tank. This allows easier clean up and higher dissolved metal loadings in the solution before it must be replaced.